In my head it seems reasonable that a high-energy bond would indicate that the bond is strong, however a question got me thinking.

Take for example lead azide, a very explosive compound that is often used as a detonator or fuse. It has a decent amount of energy stored in its bonds while being very sensitive to heat, friction, shock, etc.

This indicates to me that the lead azide has a low activation energy and therefore weak bonds, yet still plenty of energy available to be released.

  • $\begingroup$ Activation energy and heat of reaction are two seperate quantities. $\endgroup$ Apr 15 at 4:39
  • 3
    $\begingroup$ Energy isn't stored in bonds. Bonds represent a lower-energy state than the separated atoms. I.e. energetically, bonds are "valleys". Indeed, it's only because energy decreases with bonding that atoms bond at all, since bonding causes a loss of entropy. The strongest bonds are the ones that put the bonded atoms in the lowest energy state, and thus take the most energy to break. The most unstable bonds are the bonds that take the least energy to break. Compounds containing such bonds can react to release energy because these bonds can rearrange, through chemical rxns, to form stronger bonds. $\endgroup$
    – theorist
    Apr 15 at 5:09

There are two factors here to consider.

First, the energy released by a given reaction is (partly) due to the difference between the bond strength in the product vs reactant, not the absolute bond strength, unless the reaction involves just breaking a bond and not forming a new one.

Second, even if your reaction is just a bond breaking, there are multiple ways to break a bond. Reported bond enthalpies are usually defined in terms of the energy required for homolytic bond breaking, ie one electron stays on each component. For example,

$$\ce{HCl -> H. + Cl.}$$

By that measure, the H-Cl bond is quite strong, but we know that in a polar solvent that mitigates the negative effect of charge separation, the H-Cl will readily undergo heterolytic bond cleavage, where both electrons stay on one atom:

$$\ce{HCl -> H+ + Cl-}$$

In that context, we might consider the H-Cl bond to be very weak. (Note, though, that I'm also overlooking the amount of "bonding" between solvent and ions in this case, which means that it's not an example of a pure bond breaking reaction.)

All of which is to say that before you can make a statement about bond energy and bond "strength" you need to define both terms more specifically.


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